3D body motion capture is employed in a number of applications, e.g., entertainment, character animation, rehabilitation, virtual reality, gait analysis, posture and motor control, sport performance, and so on. The most commonly used form of 3D body motion capture is optical tracking. In optical tracking, several passive retro-reflective markers are attached to the main body segments of a person to track overall body motion, pose etc optically. For example, to track a user's arm, it is common to employ one marker on the hand, two on the wrist, one on the lower arm, one on the elbow, one on the upper arm, and one on the shoulder.
The passive markers are coated with a retro-reflective material to reflect light generated by strobe LEDs placed around each optical camera's lens. In this way, the position of the optical marker can be reconstructed with elementary trilateration techniques and tracked over time, provided that clear line of sight with a sufficient number of cameras is guaranteed throughout the motion capture session.
While optically-based body motion tracking can provide high accuracy drift free position tracking, it nonetheless suffers from numerous limitations. For example, the process provides a very small motion capture volume relative to the required infrastructure (e.g. about 10 cameras covering an area of 8×8 meters to guarantee an effective optical motion tracking area of only about 3×3 meters). In addition, the subject must wear several tens of markers for full body motion capture, and marker occlusion occurs frequently.
The latter problem is a fundamental limitation of the technology, which can only be slightly mitigated by over-dimensioning the system (e.g. by significantly increasing the number of cameras to reduce the frequency of marker occlusion). This, in turn, dramatically increases system cost and complexity. When marker occlusion does occur, the end-user will need to engage in laborious manual labeling of markers or, for most common use scenarios, perform for a new recording, wasting significant amount of time and resources needed to capture usable motion data.
An alternative motion capture technology is instead based on inertial sensing. This technology is usable in similar applications as those in which optical systems are used. Inertial sensing uses several inertial sensor units attached to the main body segments of the subject; for example, commercially available systems could use as many as 17 sensors for full body tracking. The primary principle in inertial motion capture technology is to integrate 3D gyroscope and 3D accelerometer signals while ensuring that the segments remain linked at their joints, to generate orientation and position data for every segment. In this way, the pose of the subject can be accurately tracked over time with high fidelity.
A significant advantage of inertial sensing is that it provides a self-contained solution which does not rely on any external infrastructure. This not only allows for motion capture in locations others than traditional laboratory settings, but it also fundamentally solves occlusion problems, simplifying the efforts and time needed by the end user to successfully achieve high quality motion capture. One disadvantage of inertial sensing based motion capture, however, is the fact of drift in absolute position tracking. The problem of drift is inherent in dead-reckoning systems since very small sensing errors can add up to a significant offset in position. Although the drift is usually very modest, for example 1% or less of traversed distance, the drift will nonetheless render the solution unsuitable for certain practical applications. For example, significant drift is unacceptable for augmented or virtual reality, or where interactions between multiple subjects are of interest.
As noted above, inertial-based body motion tracking provides numerous benefits. In an embodiment, an external position technology is used with the inertial tracking components to assist in eliminating drift. In this way, the unique benefits enabled by inertial motion tracking compared to a stand-alone optical tracking system are maintained without suffering from the inaccuracies caused by cumulative drift.
However, in order to successfully accomplish integration of such systems, certain problems and limitations need to be addressed and solved. A first fundamental problem observed by the inventors pertains to the alignment between the reference frames of the two systems. In fact, to achieve consistent motion tracking, the coordinate frames of the two systems need to be precisely aligned. Any misalignment between the systems in position or orientation will result in inconsistent motion information and the resultant detrimental effects on final performance.
A particular challenge is represented by the alignment of the heading component of the orientation (sometimes referred to as “yaw”). It is possible to stabilize the heading component of each sensing unit by using magnetometers, e.g., by referencing each inertial sensing unit's heading to magnetic North at the given geographical location. As long as the magnetic field is homogeneous in the motion capture volume, the sensing units will share a common reference heading.
In an embodiment, an external positioning system such as an optical tracking system is further used in combination with inertial body motion capture. In a typical implementation, a limited number of positional units (e.g. optical markers) are affixed to one or more inertial sensing units to allow detection and correction of drift in inertial position calculations. For example, in an implementation, a single positional unit may be affixed to a single inertial sensing unit. In a different implementation, several positional units may be used.
In a virtual reality application it may be necessary to track the subject head and appendage positions with very high accuracy. In this case, five positional units could be affixed to the inertial sensing units placed on hands, feet, and head. However, the number of positional units per subject is limited in practice, in order to preserve the benefits of the described principles as compared to a stand-alone optical tracking system.
While the use of position aiding to remove drift in the inertial position estimate will generally result in stable orientation tracking in the reference frame of the positioning system, the positioning system reference frame may not be aligned with the magnetic North reference in the motion capture volume. For this reason, when integrating an inertial motion tracking system with an external positioning aiding technology, there will generally be inconsistent headings between inertial sensing units with affixed positional units (which will have heading referenced in the positioning system coordinate frame), and sensing units without positional units (which will instead have heading referenced in the frame defined by the magnetic North in the motion capture volume).
It might be possible to estimate the difference between the two reference frames using the sensing units with affixed positional units. However, this would require a magnetic field that is homogeneous over the entire capture volume, which is generally not the case due to sources of magnetic distortion (caused by steel beams in floors/ceiling, furniture, speakers, etc.). These inconsistencies would largely nullify the potential benefits of aiding inertial based motion tracking with an accurate external positioning system.
Another problem observed by the inventors is the need to associate positional unit measurements with corresponding sensing units. While this is not a problem if a single positional unit is used (e.g. a GPS receiver or a single passive marker), or when positional units have transceivers that provide unique IDs. However, where the optical positioning system employs passive markers, the issue of associating positional units with corresponding sensing units needs to be addressed.
Moreover, the same limitation may be present in other embodiments in which inertial motion capture and external position aiding are integrated. For example, similar limitations could occur in some implementations in which an acoustic system (e.g. employing signals in the ultrasound frequency range) is used for position aiding. In some commercially available ultrasonic positioning systems, the audio channel is used only to transmit signals for precisely calculating ranges between pairs of ultrasonic transceivers, and an additional radio channel is used to communicate the transceiver ID (and sometimes to perform synchronization between the transceivers).
The use of the additional radio channel is beneficial since the audio channel capacity is relatively low, and is therefore unsuited for transmitting any additional information (as e.g. the transmitter ID) other than the signals minimally necessary for performing ranging estimation between the transceivers. However, in some implementations of these systems, in order to reduce costs, complexity, size and power consumption, it might be desired to remove the additional radio infrastructure, and use only audio transceivers placed in correspondence with some of the inertial sensing units. This solution is economical and simple, but does require special techniques to associate inertial sensing units and positional units.
While the present disclosure is directed to a system that can eliminate some of the shortcomings noted in this Background section, it should be appreciated that any such benefit is not a limitation on the scope of the disclosed principles, nor of the attached claims, except to the extent expressly noted in the claims. Additionally, the discussion of technology in this Background section is reflective of the inventors' own observations, considerations, and thoughts, and is in no way intended to accurately catalog or comprehensively summarize the prior art. As such, the inventors expressly disclaim this section as admitted or assumed prior art with respect to the discussed details. Moreover, the identification herein of a desirable course of action reflects the inventors' own observations and ideas, and should not be assumed to indicate an art-recognized desirability.